Ballasts are electronic circuits used to drive fluorescent bulbs, mercury bulbs, and arc lamps in general. Ballasts can be controlled by digital signals, for example, according to the DALI communications protocol set forth in the IEC standard of Jan. 10, 2000. According to the DALI communications protocol, a received digital signal takes the form of a frame including a start bit, a 16-bit binary word, and two end bits, giving a 19-bit frame. The 16-bit word includes, for example, an 8-bit address and an 8-bit instruction. In return, a transmitted digital signal takes the form of an 11-bit frame including a start bit, 8 bits of data, and two end bits.
The DALI communications protocol also specifies that each bit of a frame received or sent by the control circuit is encoded in the form of a biphase signal, namely in the form of a signal taking two successive states. A logic 1 is encoded as a signal (FIG. 1, ref. 110a, 110b) which is equal to 0 during a first phase and 1 during a second phase. Similarly, a logic 0 is encoded as a signal (FIG. 1, ref. 120a, 120b) equal to 1 during the first phase and 0 during the second phase. A start bit (130a, 130b) is encoded as a signal equal to 0 during a first phase and 1 during a second phase. Finally, an end bit (140a, 140b) is encoded as a signal equal to 1 during both phases.
Thus, all the bits of a frame are encoded as follows: a logic 1 is encoded by the pair of states 01; a logic 0 is encoded by the pair 10; a start bit is encoded by the pair 01; and an end bit is encoded by the pair 11. A 19-bit frame (reception) or 11-bit frame (transmission) is thus encoded as a binary number having 38 or 22 states, respectively. The frames thus encoded are transmitted at a speed of 1200 bits per second, namely 2400 states per second since each bit is encoded in two states. The transmission time for one state of a frame is thus equal to T= 1/2400, so T=416.37 μs.